1. Field of the Invention
This invention relates to a device for testing multiple degree of freedom noise attenuation structures, and in particular to a device for testing the acoustic impedance of liner designs used to attenuate noise in engine aircraft inlet and exhaust ducts.
2. Description of Related Art
Acoustic liners and similar structures used to attenuate noise in, for example, engine aircraft inlet and exhaust ducts, are conventionally formed of a solid backface and one or more perforate facesheets separated by a core structure. The core structure defines the distance between the backface and the facesheets without significantly affecting the acoustic properties of the liner.
FIG. 1 shows one such liner configuration. Liner 1 is defined as a two degree of freedom configuration because it includes two respective facesheets 2 and 3. Spacing between facesheets 2 and 3 and solid backface 4 is provided by respective cores 5 and 6.
The different spacings between the facesheets 2 and 3 and the backface 4 cause the structure to attenuate different frequencies. In the example shown, because two facesheets at two different spacings are provided, multiple frequencies of noise can be attenuated. The more facesheets or "degrees of freedom" possessed by the liner, the greater the range of acoustic frequencies attenuate.
A variety of different facesheet and core configurations are presently in use. One type of conventional liner includes a stainless steel mesh supported by an open weave perforate graphite/epoxy backface, a non-metallic core, and a structural laminate graphite/epoxy backface. A second type of known liner includes an aluminum hex core, corrosion-resistant wire mesh supported by a microporous or aluminum perforated facesheet, and a solid non-porous backface.
In order to evaluate the acoustic properties of this type of liner design, it is conventional to fabricate test samples consisting of facesheets separated by honeycomb cores of different thicknesses in order to provide varied backing depths. These samples are then tested with an acoustic impedance measurement device such as the one disclosed in U.S. Pat. No. 4,768,379. However, for each liner design evaluated, only a fixed set of face sheet/depth configurations can be tested using the conventional measurement device. In order to obtain an optimum configuration, numerous different samples must be manufactured for testing. This significantly increases the cost and time needed to design a particular multiple degree of freedom liner.
The advantages of multiple degree of freedom acoustic impedance structures include improved attenuating properties and the capability of being tuned for two or more discrete frequencies. However, these advantages are currently offset, at least partially, by the disadvantage described above that a large amount of testing is required to verify and fine tune such designs. As a result, it would be highly desirable to provide an acoustic impedance testing device in which different facesheets and depth or spacing combinations could be easily tested without the need for fabricating a different test sample for each combination to be tested.